One of the desirable quality of the AR15 platform is the ability to swap out Uppers and change to one of the many different caliber to choose from. The 300 blackout swap out you can use the same magazine and same bolt. Some of the other calibers you can swap out to you will need to change the bolt and magazine. For example the 6.5 Grendel and 6.8 SPC will not use the same bolt and magazine as the 223/5.56 will. These are becoming very popular for hunters and long range shooter that want to stay in this AR15 platform.